Photoinhibition of Chlamydomonas reinhardtii in State 1 and State 2 - Damages to the photosynthetic apparatus under linear and cyclic electron flow

The relationship between state transitions and photoinhibition has been stu died in Chlamydomonas reinhardtii cells. In State 2, photosystem II activit y was more inhibited by light than in State 1. In State 2, however, the D1 subunit was not degraded, whereas a substantial degradation was observed in State 1, These results suggest that photoinhibition occurs via the generat ion of an intermediate state in which photosystem II is inactive but the D1 protein is still intact. The accumulation of this state is enhanced in Sta te 2, be cause in this State only cyclic photosynthetic electron transport is active, whereas there is no electron flow between photosystem II and the cytochrome b(6)f complex (Finazzi, G., Furia, A., Barbagallo, R, P,, and F orti, G, (1999) Biochim. Biophys. Acta 1413, 117-129). The activity of phot osystem I and of cytochrome b(6)f as well as the coupling of thylakoid memb ranes was not affected by illumination under the same conditions. This allo ws repairing the damages to photosystem II thanks to cell capacity to maint ain a high rate of ATP synthesis (via photosystem I-driven cyclic electron flow). This capacity might represent an important physiological tool in pro tecting the photosynthetic apparatus from excess of light as well. as from other a-biotic stress conditions.